Distributed optical sensing technology is proving to be suitable for a number of downhole oil and gas applications ranging from temperature sensing to passive seismic monitoring. One particularly advantageous aspect of this technology is that it enables the downhole components of the system to be passive, i.e., the electronics can be kept at the surface and not in the wellbore. As the technology evolves to develop new and improved systems with increased performance and sensitivity, certain obstacles have been encountered. For example, fiber optic distributed sensing techniques often rely on (but do not necessarily require) monitoring the backscattered component of light injected into an optical fiber. The properties of this backscattered light (e.g., phase, spectrum, amplitude, etc.) can provide access to various downhole parameters, such as the temperature at a specific location of the fiber. However, as the length of the fiber increases, the injected light and backscattered light suffers from increased attenuation from various loss mechanisms. This attenuation can be compensated through the use of a high power laser source for light injection. However, this approach is limited since for every fiber there exists a threshold value of laser power beyond which nonlinear behavior starts to add unwanted features. Therefore, very weak signals can be commonplace in some distributed optical sensing systems, and extracting information from the signals is a challenging task.
When working with very weak signals, measurement noise such as noise introduced by the sensor, receiver electronics, and other unavoidable noise sources, becomes a prominent issue. For very long optical fibers, such as those deployed in ultra-deep/ultra-long oil and gas boreholes, the signals could be very weak (not more than few photons). Existing systems cannot perform adequately in this regime as they lack proper handling and understanding of weak signals and/or noise. Weak signals at the level of a few photons (e.g., the noise as well as the signal) acquire non-classical (Quantum) properties.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.